South Korea fired warning shots Tuesday after an unidentified object flew south from rival North Korea, Seoul’s military said. Local media said it may have been a North Korean military drone.

South Korea’s Joint Chiefs of Staff said in a statement that the military broadcast a warning to North Korea in response to the object before firing the warning shots. It said the military also bolstered its air surveillance. The statement provided no other details.There was no immediate comment in North Korea’s state-run media, and no reports that the North had returned fire.

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South Korea’s Yonhap news agency, without citing a source, reported that the South fired about 90 machine gun rounds into the air and toward North Korea. It said South Korea was analyzing whether a North Korean military drone had crossed the border.

The Koreas face off across the world’s most heavily armed border, and the two sides occasionally clash. North Korea is also building nuclear-tipped missiles and has greeted new South Korean President Moon Jae-in, who wants to reach out to the North, with two missile test-launches, part of a flurry of tests since leader Kim Jong Un took over in late 2011.

In 2014, the rivals traded machine gun and rifle fire after South Korean activists released anti-North Korean propaganda balloons across the Demilitarized Zone that bisects the Korean Peninsula, but no casualties were reported.

Attacks blamed on North Korea in 2010 killed 50 South Koreans.

North Korean drone flights across the border are unusual, but have happened before. In January 2016, South Korea said its military fired 20 machine gun warning shots after a North Korean drone briefly crossed the border.

North Korea has in recent years touted its drone program, a relatively new addition to its arsenal. In 2013, state media said North Korean leader Kim watched a drone attack drill on a simulated South Korean target.

In 2014, South Korean officials discovered what they described as several North Korean drones that had flown across the border. Those drones were crude and low-tech, but were still considered a potential new security threat.